IIS? That's an important detail when asking for help as most Moodle servers are Apache or Nginx.
Is this a new server? Or a new website in IIS? IIS has a default transfer limit of 30000000 bytes, i.e. just under 30 MB, set in system.webServer/security/requestFiltering/requestLimits/maxAllowedContentLength.
This can be set at server level as well as for individual websites.
